The present invention relates to optical multilayer films and reflective screens, and more particularly relates to screens on which projected images from projection-type projectors, such as, video projectors, film projectors, and overhead projectors, are displayed. In particular, the present invention relates to optical multilayer films which reflect selected wavelengths, which provide high-contrast images projected by projector light even in a bright environment, and in which high color reproducibility is achieved; and screens including such optical multilayer films.
Recently, overhead projectors and slide projectors have widely been used to present information in meetings and the like. Furthermore, video projectors and motion picture film projectors including liquid crystal have become widely used in ordinary homes. In these projectors, light from a light source is modulated, for example, by a transmissive liquid crystal panel, to form image light, and the image light is projected onto a screen through an optical system, such as a lens.
For example, a front projector capable of forming a color image on a screen includes an illuminating optical system which separates light emitted from a light source into red (R), green (G), and blue (B) color beams and converges each color beam into a predetermined light path; a liquid crystal panel (light valve) which modulates the respective color beams of R, G, and B separated by the illuminating optical system; and a light-synthesizing unit which synthesizes the respective color beams of R, G, and B modulated by the liquid crystal panel. A color image synthesized by the light-synthesizing unit is enlarged and projected on a screen by a projector lens.
Recently, another type of projector has been developed in which a narrow-band three primary color light source is used and the respective color beams of R, G, and B are spatially modulated by a grating light valve (GLV) instead of a liquid crystal panel.
In the projectors described above, projection screens are used for displaying projected images. The projection screens are broadly classified into two types, i.e., a transmissive type in which projected light is emitted from the rear side of the screen and the image is viewed from the front side of the screen, and a reflective type in which projected light is emitted from the front side of the screen and the projected light reflected from the screen is viewed. In either type, it is necessary to produce bright, high-contrast images in order to achieve a screen with high visibility.
However, in front projectors, unlike light self-emitting displays and rear projectors, it may be impossible to reduce reflection of external light, for example, using a neutral density (ND) filter, and it is difficult to increase the contrast on the reflective screen in a bright environment.
In order to overcome such a problem, a reflective screen has been proposed which includes an optical thin film composed of a dielectric multilayer film (optical multilayer film) which has a high reflection characteristic with respect to light in a specific wavelength region and a high transmission characteristic with respect to at least light in the visible wavelength region other than the light in the specific wavelength region. The thicknesses of the individual optical layers in the dielectric multilayer film are designed according to a simulation based on a matrix method. For example, refer to Japanese Unexamined Patent Application Publication No. 2003-270725.
In the screen described above, the optical multilayer film serves as a band-pass filter, and most of light in a specific wavelength region is reflected by the action of the optical multilayer film. Furthermore, for example, when external light is incident on the screen, most of the light other than the light in the specific wavelength region is transmitted through the optical thin film and is not substantially reflected.
As described above, in the reflective screen, only light with a specific wavelength can be selectively reflected, and reflection of external light can be relatively suppressed compared with the commonly used screens. Consequently, the contrast of the image formed on the screen can be prevented from decreasing and reflection of external light can be effectively reduced. Thereby, a bright image can be produced. Furthermore, in the reflective screen, even if the projection environment is bright, a clear image can be produced. Thus, it is possible to obtain a clear image without being affected by the brightness of the environment. In particular, in the case when a light source, such as a GLV, has a steep spectrum, and the full width at half-maximum of the spectrum of the light source is narrow relative to the full width at half-maximum of the reflectance profile in a specific wavelength region of the screen, significantly high contrast can be achieved, and the capability of the light source can be fully utilized.
However, even if the reflective screen described above is used, in the case of a LCD projector using a high-pressure mercury lamp (UHP lamp), the white balance of image light becomes greatly disturbed, giving rise to a problem.
It is desirable to provide an optical multilayer film in which the chromaticity of reflected light is adjusted and uniform color representation is achieved over the entire display, and a reflective screen including the optical multilayer film.